1. Field of the Invention
This invention relates to a signal transmission circuit having a latch-up function in which signals are transmitted between an input and an output terminals, which are electrically insulated to each other.
2. Description of the Prior Art
In a signal transmission circuit which requires electric insulation between its input and output terminals, such as a solid state relay (referred to as SSR, below), the output part of the circuit is comprised of transistors, thyristers, two-way thyristers (triak), MOSFETs, or the like. In such SSR circuits, a thyrister coupler 1 shown in FIG. 6 and a triac coupler 2 shown in FIG. 7 have a latch-up function in which an output signal is kept on when the input signal has been turned off.
In the coupler shown in FIGS. 6 or 7, the light from a photo-diode 3, which is provided in its input part, triggers thyrister 4 or triac 5 which is provided in its output part, thus allowing thyrister 4 or triac 5 to conduct. This results in a continuous current flow in a load 7 connected between an output terminal 6 and a high voltage supply V.sub.cc. Load 7 is, thus, driven by the current.
In said couplers, their noise resistance (dv/dt) for a surge current is not high enough, so that a snubber circuit 8, which includes a resistor and a capacity, must be connected with output terminal 6 in order to increase the noise resistance.
In said couplers, however, a leakage current flows through snubber circuit 8 connected with output terminal 6, even in an ordinary state. This leakage current badly affects the circuit operation. Further, although a snubber circuit is provided in the circuit, there is a limit in its noise resistance. This circuit cannot, therefore, be used in a circuit which requires a high noise resistance.
As described above, the prior art SSR circuits having a latch-up function require external structures to improve their noise resistance. These external structures, however, generate a leakage current which badly affects the circuit operation. Although said external structures to improve the noise resistance are provided in said SSR circuit, there is still a limit in ability of improving the noise resistance of this circuit.